Cook the angel hair pasta according to package directions. Drain, rinse and set aside.
Add the broccoli to a large skillet with about 1/2 cup water. Cover and steam over medium heat until bright green and tender, adding more water as needed. Remove and set aside.
Add the mushrooms to the skillet and sauté until softened and lightly browned, adding a splash of water if needed. Remove and set aside.
Add the garlic with about 1/4 cup water and cook on low heat for about 5 minutes until softened, adding additional water if needed. Do not brown.
Add the 1 cup water, lemon juice and nutritional yeast. Stir and simmer for a few minutes.
If you’d like the sauce thicker and more clingy to the noodles, mix the cornstarch or potato starch with 1 tablespoon cold water until completely dissolved (this is called a “slurry”). Lower heat. Stir “slurry” into the sauce and cook another 1–2 minutes until slightly thickened. This happens fast.
Add the cooked pasta to the sauce and toss well. Return the mushrooms and broccoli to the pan and stir gently. Fold in the spinach until wilted.
Season with salt, black pepper and crushed red pepper flakes.